Overview

Brief Summary

The Arctic Tern (Sterna paradisaea) is perhaps best known for its extraordinary annual migration, which is often cited as the longest seasonal distance traveled by any animal. It has long been known that the Arctic Tern breeds in the Arctic and migrates each year to spend the northern winter at high latitudes in the Southern Ocean. Until recently, what has been known about the Arctic Tern's migration has come from limited banding recoveries and at-sea observations. Thanks to new technology, however, far more detailed data on this small (<125 grams) bird's annual migration are now available. Egevang et al. (2010) fitted 11 Arctic Terns with miniature (1.4 gram) geolocators. They found that the annual distances traveled are, in fact, even greater than previously believed--more than 80,000 km annually for some individuals. All tracked terns migrated south to spend the austral summer (December–March) south of 58° S and between 0 and 61° W in the Atlantic sector of the Southern Ocean. This region, which includes the Weddell Sea, is particularly productive, and supports higher densities of a key prey for many seabirds, Antarctic krill (Euphausia superba), than do other regions of the Southern Ocean. All birds began the return migration to breeding colonies in early–mid April, always traveling over deep water at considerable distance from continental shelf margins.

Egevang et al. note that the routes used for both the southbound and northbound migrations showed a high level of congruence with parts of those taken by Manx Shearwater (Puffinus puffinus) and Cory’s Shearwater (Calonectris diomedea), which also winter in the South Atlantic (although considerably farther north than the Arctic Tern). Thus, despite their small size, Arctic Terns apparently exploit the prevailing global wind systems (clockwise in the North, and counter-clockwise in the South Atlantic) much as the substantially larger shearwaters do, as has been previously suggested. These new geotracking studies found that the main wintering region was the marginal ice zone around Antarctica, which agrees with at-sea observations. The mean duration of the northbound migration was about 40 days (range 36 to 46). Mean duration of the southbound migration was about 93 days (range 69 to 103). (Egevang et al. 2010)

Perhaps most striking, the tracked birds were found to travel nearly twice the total distance generally cited for the annual Arctic Tern migration (typically quoted as ~ 40,000 km). Given that Arctic Terns can live for more than 30 years, the total distance traveled in a lifetime may exceed 2.4 million km, equivalent to approximately three round-trip journeys to the Moon. (Egevang et al. 2010)

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Arctic terns are the champion of all migrating birds. They migrate the furthest south to overwinter, as far as the South Pole ice. In the spring, they return to their breeding grounds in the North Sea area or further northwards. In total, they make an annual trip of around 35,000 kilometers per year! A young Arctic tern supplied with a transmitter in Groningen in the summer of 2011 flew at least 90,000 kilometers in 273 days. If you realize that Arctic terns can reach an age of 30 years and make this trip every year, than this tern possibly flies more than 2.7 million km in its entire life. That is equivalent to approximately 3 return trips to the moon!

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Description

The Arctic tern is a long-distance migrant, making a staggering annual round-trip from its Arctic or northern temperate breeding range to the Antarctic where it spends winter (3). This is probably the longest migration undertaken by any bird (2) and means that the Arctic tern sees more sunlight each year than any other animal, as they experience a 'second summer' by travelling south in winter (5). They are very similar in appearance to the common tern (Sterna hirundo), so much so that birdwatchers call unidentified terns 'commic terns', an amalgamation of the two common names (6). Arctic terns are slightly smaller than common terns, and have a shorter bill and longer tail (2). The rump is white, the underparts are darker and the wing lacks the dark wedge on the outer edge, which is a key identifying feature of common terns (6) . During summer, the bill becomes bright red and lacks the black tip seen in common terns (2). Long tail streamers also develop in summer (2). A 'kee-arr' alarm call and a piping 'pi-pi-pi-pi-pi' call are produced (2) (6).

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Biology

The Arctic tern tends to arrive back in its breeding habitats after the long migration from May to June (3). Males court females with a 'fish flight', an impressive aerial display which culminates in the male presenting the female with a 'gift' in the form of a fish (6). Nesting tends to occur in a hollow on the ground at a good distance from the shore in short vegetation (4) (6). Pairs, which mate for life, produce between one and three eggs, which are incubated for up to 24 days (3) (6). After a further 24 days or so, the chicks will have fully fledged. The males defend the nests if they are threatened, by diving at potential predators and livestock (6). Arctic terns are very long-lived species, with a maximum recorded life-span of 29 years (3). This species feeds mainly on fish, particularly sand eels (3) (4), which they catch by making short dives into the water (6). Feeding tends to occur within 3 km of the breeding colony, although they have been recorded to travel 10 km away in order to feed (4). When the breeding season is over, these birds head off once more on their long journey south (3).

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Distribution

Range Description

The Arctic Tern has a circumpolar range, breeding in the Arctic and subarctic regions of Europe, Asia and North America as far south as Brittany, France and Massachusetts (USA). It is a transequatorial migrant, and can be found wintering throughout the Southern Ocean to the edge of the Antarctic ice and the southern tips of South America and Africa (del Hoyo et al. 1996). Overall population trends are relatively unknown, though the 2008 breeding season in the north of the United Kingdom was reported to be a failure by the RSPB.

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Circumpolar; nests on Northern European islands and peninsulas from Iceland to Northern Russia/Siberia, British Isles, Netherlands, Denmark, Sweden, the Baltic Nations, Northern Alaska, extreme of Northern Canada, Greenland, Newfoundland, and south along Atlantic Coast to Massachussetts; winters in S. Hemisphere in subantarctic and Antarctic waters of Atlantic, Pacific, and Indian Oceans.

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Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) Breeding range extends from northern Alaska east to northern Ellesmere Island, south to the Aleutian Islands, northwestern British Columbia, northwestern Saskatchewan, northern Ontario, New Brunswick, and along Atlantic coast, locally to Maine and Massachusetts; a disjunct colony exists in Puget Sound, Washington, and solitary nesters in northcentral Montana. Outside North America, nesting occurs in Greenland on all coasts; in the Palearctic north to Iceland, Svalbard, and Franz Josef Land, and south to the Netherlands and sparsely in Belgium, Ireland, and northwestern France; and in northern Russia and widely along Russian far eastern arctic coasts (Wrangel Island, Chukotska and Kamchatka Peninsulas south to Sakhalin Island) (Hatch 2002).

During the nonbreeding season, this species occurs primarily in the antarctic and subantarctic regions of the Southern Hemisphere, with small numbers reported throughout the year from Namibia to Mozambique, southern Australia, and New Zealand (Hatch 2002). Migrants occur widely in areas between the breeding areas and antarctic region.

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Range

Arctic terns breed around the Arctic and temperate northern parts of the northern Hemisphere. They travel massive distances to overwinter in the Southern Ocean and Antarctica (4). The population that breeds in Britain and Ireland is at the southern-most limit of its breeding range. This range extends through Greenland and Iceland, along the Baltic and Scandinavian coasts reaching into Siberia. Most of the British breeding population occurs in Scotland, Orkney, Shetland and the Hebrides. Populations also occur in England in the northwest and northeast and there are small numbers in Norfolk and around the southern coast; they also breed in Anglesey in Wales (4).

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Physical Description

Morphology

Arctic terns are 14-17''(36-43 cm) long with a wingspread of 29-33''(74-84 cm). They are white with black caps and gray mantles, and a deeply-forked tail. In spring and summer, the entire bill is blood-red. Their legs are so short that the birds appear to be crouched when standing.

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Ecology

Habitat

Habitat and Ecology

Behaviour The species is a very strong migrant and makes exceptional long-distance movements offshore or along western continental coastlines (del Hoyo et al. 1996, Melville and Shortridge 2006) between its high Arctic breeding grounds and Antarctic wintering grounds (del Hoyo et al. 1996). It breeds between May and July (although the exact timing varies with temperature and food availability) in solitary pairs or colonies of a few to several hundred pairs (usually 2-25) (del Hoyo et al. 1996), and remains gregarious throughout the year especially when roosting, foraging (Snow and Perrins 1998) and on passage (Higgins and Davies 1996). The species generally feeds within 3 km of breeding colonies but may occasionally forage up to 50 km away (del Hoyo et al. 1996). On its wintering grounds in Antarctica it may also forage in association with Antarctic Minke Whale Balaenoptera bonaerensis in the open ocean north of the pack-ice zone (Higgins and Davies 1996). HabitatBreeding The species breeds along northern coastlines (del Hoyo et al. 1996)and on inshore islands (Flint et al. 1984, Snow and Perrins 1998) as well as inland on tundra and forest-tundra (Flint et al. 1984). It shows a preference for habitats with a vegetation cover of less than 40 %, nesting on sand or shingle beaches, ridges (del Hoyo et al. 1996) and spits (Flint et al. 1984), rocky ground (Flint et al. 1984, Richards 1990, del Hoyo et al. 1996) and small islands (Flint et al. 1984, del Hoyo et al. 1996, Snow and Perrins 1998) in lakes and coastal lagoons (del Hoyo et al. 1996). It may also nest on islets or banks along rivers (Snow and Perrins 1998), on swampy tundra (Flint et al. 1984, del Hoyo et al. 1996) and peatlands with bog hummocks (del Hoyo et al. 1996) and reed-covered flats (Flint et al. 1984), or on inland heaths, rough pastures (del Hoyo et al. 1996, Snow and Perrins 1998), meadows (del Hoyo et al. 1996) and sedge grassland (Snow and Perrins 1998) not far from water (Flint et al. 1984). The species also forages offshore, in ice-filled coastal bays or over wet tundra (del Hoyo et al. 1996). Non-breeding On passage it largely flies over open ocean (Snow and Perrins 1998) resting at sea on kelp, logs or flotsam, but may occur inland or along coastlines on beaches, reefs and spits (Higgins and Davies 1996). During the winter the species is pelagic, foraging at the edges of pack-ice, icebergs and ice-floes near shore (especially in channels between ice-floes) (Higgins and Davies 1996) and up to 24 km offshore (Higgins and Davies 1996, del Hoyo et al. 1996) often in association with Antarctic Minke Whale Balaenoptera bonaerensis (Higgins and Davies 1996). It also roosts on ice-floes and icebergs during this season (Higgins and Davies 1996). Diet Its diet consists predominantly of fishas well as crustaceans (especially planktonic species), molluscs, insects (e.g. caterpillars, Chironomidae) and earthworms (del Hoyo et al. 1996). It will also take berries in the early spring on arrival on its breeding grounds but does not readily switch to other prey items when preferred prey supplies fail (del Hoyo et al. 1996). Breeding site The nest is a shallow scrape (del Hoyo et al. 1996) in sand, shingle or turf (Richards 1990) on beaches, ridges (del Hoyo et al. 1996) and spits (Flint et al. 1984), rocky ground (Flint et al. 1984, Richards 1990, del Hoyo et al. 1996), small islands (Flint et al. 1984, del Hoyo et al. 1996, Snow and Perrins 1998) in lakes, coastal lagoons (del Hoyo et al. 1996) and rivers (Snow and Perrins 1998), swampy tundra (Flint et al. 1984, del Hoyo et al. 1996) and peatlands with bog hummocks (del Hoyo et al. 1996) and reed-covered flats (Flint et al. 1984), or on inland heaths, rough pastures (del Hoyo et al. 1996, Snow and Perrins 1998), meadows (del Hoyo et al. 1996) and sedge grassland (Snow and Perrins 1998) not far from water (Flint et al. 1984). It will also nest on artificial structures (del Hoyo et al. 1996). Management information Removing feral American mink Neovison vison from a large archipelago with many small islands in the Baltic Sea resulted in an increase in the breeding density of this species in the area (Nordstrom et al. 2003). Gull control measures may also be practised successfully at some sites to reduce predation and displacement, especially when carried out in conjunction with the use of recordings and models to induce recolonistion of nesting terns (Buckley and Buckley 1984).

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Comments: NON-BREEDING: mostly pelagic, rarely in coastal bays and estuaries. Migrates primarily at sea along coasts. BREEDING: Nests on ground on rocky, sandy, gravelly, or grass-covered coasts and islands, in far north on islands in lakes and ponds and in marshes and on riverine gravel bars, sometimes on open tundra (Terres 1980). See Spendelow and Patton (1988) for further details.

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In Britain, this species breeds around the coast (4) in open sand or shingle habitats or in moorland and coastal heathland (3). As they are vulnerable to predation, they often breed on offshore islands areas where there are no mammal predators such as rats and mink (3). In winter this species stays out at sea, resting on floating objects and ice (3).

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Migration

Non-Migrant: No. All populations of this species make significant seasonal migrations.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: Yes. At least some populations of this species make annual migrations of over 200 km.

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Trophic Strategy

Arctic terns hover 30-40 feet over the water on beating wings and then dive suddenly into the water with a splash, often completely submerging to catch small fishes such as capelin, sand launae, sand eel, and small crustaceans.

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Global Abundance

Comments: Global breeding population is estimated at between 1 to 2 million pairs; includes primarily coastal estimates of about 12,800 pairs on the Atlantic U.S. coast; 80,000 pairs in Greenland; 200,000-500,000 pairs in Iceland; 72,000 pairs in Scotland; 70,000 pairs elsewhere in Atlantic Europe; 50,000 pairs in Baltic European countries; several hundred thousand in Russia; and several hundred thousand in Alaska (Lensink 1984, Hatch 2002). Few estimates exist for interior-nesting birds.

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Lifespan, longevity, and ageing

Maximum longevity: 34 years (wild) Observations: As with other terns, these animals are long-lived. No senescence has been demonstrated in the Artic tern (Roger Gosden 1996). A 34-year-old individual was released and appeared in excellent health (John Terres 1980).

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Reproduction

They nest in colonies defended by the males in the rocky or sandy beaches of the far north. The nest usually consists of a hollow in sand, gravel or moss. In June-July, 2-3 brown or greenish eggs with brown speckles are incubated for 21-22 days. Young fly about 21-28 days after hatching

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Lays clutch of 2-3 eggs, June-July. Incubation, by both sexes, 20-24 days. Young are tended by both parents, may leave nest soon after hatching but remain nearby, first fly at 20-23 days (then still fed by parents). Nests usually in small scattered colonies, in large dense colonies only at southern edge of range.

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First breeds at 3-4 years old. Nests are built by both sexes on open ground among colonies. 1-3 eggs, incubated by both partners for 20-24 days. Young are fed by both parents. First flight occurs around 21-28 days old.

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IUCN Red List Assessment

This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

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Foxes, raccoons, weasels, rats, gulls, and other seabirds are all predators of terns and their eggs. Massive spraying of marshes with insecticides (DDT) for mosquito control has killed many terns through their consumption of DDT-laden minnows. In the last decade of the 19th century and in the first decade of the present one, plume hunters killed tens of thousands of terns for their plumage for women's hats.

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Status

Included in the Birds of Conservation Concern Amber List (medium conservation concern) (3). Receives general protection in Great Britain under the Wildlife and Countryside Act and listed under Annex I of the EC Birds Directive. Listed as a Species of European Conservation Concern (4).

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Trends

Population

The global population is estimated to number > c.2,000,000 individuals (Wetlands International 2006), while the population in Russia has been estimated at c.10,000-100,000 breeding pairs and c.1,000-10,000 individuals on migration (Brazil 2009).

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Threats

The species is potentially threatened by climate change because it has a geographically bounded distribution: its global distribution is restricted to within c.10o latitude from the polar edge of continent and within which 20-50% of current vegetation type is projected to disappear under doubling of CO2 levels (Birdlife International, unpublished data).

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Comments: This species is vulnerable to food shortages, predation (including egg predation by humans), pollution, and habitat degradation and loss (Howes and Montevecchi 1993). Disturbance at colony sites could cause desertion or declines (Howes and Montevecchi 1993), but terns have also shown potential for habituation to human activity under certain conditions. Growing gull populations have displaced terns from breeding habitat in some parts of Atlantic Canada and increased predation on young and eggs (Lock 1992). Climatic warming could decrease abundance or change distribution of ice-associated prey species such as the Arctic cod.

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This species is listed on the Birds of Conservation Concern Amber List; its population has experienced periodic declines since the 1980s and low breeding success throughout the 1990s (4). Declines have also occurred elsewhere in Europe. The main threats facing this species include nest predation by hedgehogs, introduced American mink (Mustela vison) and rats, as well as coastal development, disturbance by recreational activities, and a lack of the most important food source, sandeels, possibly caused by over-fishing by humans (4).

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Management

Management Requirements: Gull control has been beneficial for this species in Maine (Buckley and Buckley 1984). See Minsky (1981) for discussion of tern management on Cape Cod.

Biological Research Needs: Very little is known about nonbreeders in the antarctic, where most of the mortality occurs. Better information is needed on wintering ecology in the southern hemisphere and threats to breeding populations in the northern hemisphere.

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Wikipedia

Arctic tern

The Arctic tern (Sterna paradisaea) is a seabird of the ternfamilySternidae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe, Asia, and North America (as far south as Brittany and Massachusetts). The species is strongly migratory, seeing two summers each year as it migrates along a convoluted route from its northern breeding grounds to the Antarctic coast. Recent studies have shown average annual roundtrip lengths of about 70,900 km (44,100 mi) for birds nesting in Iceland and Greenland[3] and c. 90,000 km (56,000 mi) for birds nesting in the Netherlands.[4] These are by far the longest migrations known in the animal kingdom. The Arctic tern flies as well as glides through the air, performing almost all of its tasks in the air. It nests once every one to three years (depending on its mating cycle); once it has finished nesting it takes to the sky for another long southern migration.

Arctic terns are medium-sized birds. They have a length of 28–39 cm (11–15 in) and a wingspan of 65–75 cm (26–30 in).[5] They are mainly grey and white plumaged, with a red beak (as long as the head, straight, with pronounced gonys) and feet, white forehead, a black nape and crown (streaked white), and white cheeks. The grey mantle is 305 mm, and the scapulae are fringed brown, some tipped white. The upper wing is grey with a white leading edge, and the collar is completely white, as is the rump. The deeply forked tail is whitish, with grey outer webs. The hindcrown to the ear-coverts is black.

Arctic terns are long-lived birds, with many reaching thirty years of age. They eat mainly fish and small marine invertebrates. The species is abundant, with an estimated one million individuals. While the trend in the number of individuals in the species as a whole is not known, exploitation in the past has reduced this bird's numbers in the southern reaches of its range.

The Arctic tern was known as sea swallow describing their slender shape as they swoop over the water.

The Arctic tern has a continuous worldwide circumpolar breeding distribution; there are no recognized subspecies. It can be found in coastal regions in cooler temperate parts of North America and Eurasia during the northern summer. While wintering during the southern summer, it can be found at sea, reaching the northern edge of the Antarctic ice.[6]

The Arctic tern is famous for its migration; it flies from its Arctic breeding grounds to the Antarctic and back again each year, the shortest distance between these areas being 19,000 km (12,000 mi). The long journey ensures that this bird sees two summers per year and more daylight than any other creature on the planet.[7] One example of this bird's remarkable long-distance flying abilities involves an Arctic tern ringed as an unfledged chick on the Farne Islands, Northumberland, UK, in the northern summer of 1982, which in October 1982, just three months from fledging, reached Melbourne, Australia. Assuming a direct route of flight, the distance covered would have been more than 22,000 km (14,000 mi).[8] Another example is that of a chick ringed in Labrador, Canada, on 23 July 1928. It was found in South Africa four months later.[9]

A 2010 study using tracking devices attached to the birds showed that the above examples are not unusual for the species. In fact, it turned out, previous research had seriously underestimated the annual distances travelled by the Arctic tern. Eleven birds that bred in Greenland or Iceland covered 70,900 km (44,100 mi) on average in a year, with a maximum of 81,600 km (50,700 mi). The difference from previous estimates is due to the birds' taking meandering courses rather than following a straight route as was previously assumed. The birds follow a somewhat convoluted course in order to take advantage of prevailing winds.[3] The average Arctic tern lives about twenty years, and will, based on the above research, travel some 2.4 million km (1.5 million mi) during its lifetime.

A 2013 tracking study of half a dozen Arctic terns breeding in the Netherlands[4] shows average annual migrations of c. 90,000 km (56,000 mi). On their way south, these birds roughly followed the coastlines of Europe and Africa.[10] Having rounded the southern tip of Africa, they then turned east, some flying approximately halfway to Australia before again turning south to eventually reach Wilkes Land in the north-eastern Antarctic. One bird flew several hundred kilometres along the south coast of Australia before turning south for the Antarctic, while one flew along the entire south coast of Australia, passing between Australia and Tasmania. Having reached the Melbourne area, it turned south and flew in an arc to Wilkes Land in the north-east Antarctic, passing the south-western tip of New Zealand's South Island en route. Once back in the Netherlands, this bird had journeyed c. 91,000 km (57,000 mi), the longest migration yet recorded for any animal.

Arctic terns usually migrate sufficiently far offshore that they are rarely seen from land outside the breeding season.[11]

The Arctic tern is a medium-sized bird around 33–36 cm (13–14 in) from the tip of its beak to the tip of its tail. The wingspan is 76–85 cm (30–33 in).[11] The weight is 86–127 g (3.0–4.5 oz). The beak is dark red, as are the short legs and webbed feet. Like most terns, the Arctic tern has high aspect ratio wings and a tail with a deep fork.[11]

The adult plumage is grey above, with a black nape and crown and white cheeks. The upperwings are pale grey, with the area near the wingtip being translucent. The tail is white, and the underparts pale grey. Both sexes are similar in appearance. The winter plumage is similar, but the crown is whiter and the bills are darker.[11]

An Arctic tern in flight with wings spread

Juveniles differ from adults in their black bill and legs, "scaly" appearing wings, and mantle with dark feather tips, dark carpal wing bar, and short tail streamers.[11] During their first summer, juveniles also have a whiter forecrown.[12]

The species has a variety of calls; the two most common being the alarm call, made when possible predators (such as humans or other mammals) enter the colonies, and the advertising call.[13] The advertising call is social in nature, made when returning to the colony and during aggressive encounters between individuals. It is unique to each individual tern and as such it serves a similar role to the bird song of passerines, identifying individuals. Eight other calls have been described, from begging calls made by females during mating to attack calls made while swooping at intruders.

While the Arctic tern is similar to the common and roseate terns, its colouring, profile, and call are slightly different. Compared to the common tern, it has a longer tail and mono-coloured bill, while the main differences from the roseate are its slightly darker colour and longer wings. The Arctic tern's call is more nasal and rasping than that of the common, and is easily distinguishable from that of the roseate.[14]

This bird's closest relatives are a group of South Polar species, the South American (Sterna hirundinacea), Kerguelen (S. virgata), and Antarctic (S. vittata) terns.[15] On the wintering grounds, the Arctic tern can be distinguished from these relatives; the six-month difference in moult is the best clue here, with Arctic terns being in winter plumage during the southern summer. The southern species also do not show darker wingtips in flight.

The immature plumages of Arctic tern were originally described as separate species, Sterna portlandica and Sterna pikei.[16]

Breeding begins around the third or fourth year.[17] Arctic terns mate for life and, in most cases, return to the same colony each year.[18]Courtship is elaborate, especially in birds nesting for the first time.[19] Courtship begins with a so-called "high flight", where a female will chase the male to a high altitude and then slowly descend. This display is followed by "fish flights", where the male will offer fish to the female. Courtship on the ground involves strutting with a raised tail and lowered wings. After this, both birds will usually fly and circle each other.[19]

An Arctic tern chick on the Farne Islands, Northumberland, England

Both sexes agree on a site for a nest, and both will defend the site. During this time, the male continues to feed the female. Mating occurs shortly after this.[19] Breeding takes place in colonies on coasts, islands and occasionally inland on tundra near water. It often forms mixed flocks with the common tern. It lays from one to three eggs per clutch, most often two.[11]

An Arctic tern protecting nest near Markarfljot river in south Iceland

It is one of the most aggressive terns, fiercely defensive of its nest and young. It will attack humans and large predators, usually striking the top or back of the head. Although it is too small to cause serious injury to an animal of a human's size, it is still capable of drawing blood, and is capable of repelling many raptorial birds and smaller mammalian predators such as foxes and cats.[7] Other nesting birds, such as alcids, often incidentally benefit from the protection provided by nesting in an area defended by Arctic terns.

The nest is usually a depression in the ground, which may or may not be lined with bits of grass or similar materials. The eggs are mottled and camouflaged.[11] Both sexes share incubation duties. The young hatch after 22–27 days and fledge after 21–24 days.[11] If the parents are disturbed and flush from the nest frequently the incubation period could be extended to as long as 34 days.[13]

When hatched, the chicks are downy. Neither altricial nor precocial, the chicks begin to move around and explore their surroundings within one to three days after hatching.[20] Usually they do not stray far from the nest. Chicks are brooded by the adults for the first ten days after hatching.[21] Both parents care for hatchlings.[11] Chick diets always include fish, and parents selectively bring larger prey items to chicks than they eat themselves.[13] Males bring more food than females. Feeding by the parents lasts for roughly a month before being weaned off slowly.[11] After fledging, the juveniles learn to feed themselves, including the difficult method of plunge-diving.[22] They will fly south to winter with the help of their parents.[23] National Geographic calculated that during the lifespan of 30 years, an Arctic Tern would have migrated over 2.4 Million Kilometers, the equivalent of traveling from Earth to the Moon over 3 times.

The diet of the Arctic tern varies depending on location and time, but is usually carnivorous. In most cases, it eats small fish or marine crustaceans.[6][11] Fish species comprise the most important part of the diet, and account for more of the biomass consumed than any other food. Prey species are immature (1–2-year old) shoaling species such as herring, cod, sandlances, and capelin.[7] Among the marine crustaceans eaten are amphipods, crabs and krill. Sometimes, these birds also eat molluscs, marine worms, or berries, and on their northern breeding grounds, insects.[20]

Arctic terns sometimes dip down to the surface of the water to catch prey close to the surface. They may also chase insects in the air when breeding.[20] It is also thought that Arctic terns may, in spite of their small size, occasionally engage in kleptoparasitism by swooping at birds so as to startle them into releasing their catches.[20] Several species are targeted—conspecifics, other terns (like the common tern), and some auk and grebe species.[13]

While nesting, Arctic terns are vulnerable to predation by cats and other animals.[6] Besides being a competitor for nesting sites, the larger herring gull steals eggs and hatchlings. Camouflaged eggs help prevent this, as do isolated nesting sites.[22] While feeding, skuas, gulls, and other tern species will often harass the birds and steal their food.[26] They often form mixed colonies with other terns, such as common and Sandwich terns.

At the southern part of their range, the Arctic tern has been reducing in numbers. Much of this is due to lack of food.[12] However, most of these birds' range is extremely remote, with no apparent trend in the species as a whole.[20]

Birdlife International has considered the species to be at lower risk since 1988, believing that there are approximately one million individuals around the world.[2]

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Names and Taxonomy

Taxonomy

Comments: Monotypic; no subspecies are recognized. Protein electrophoresis and phenetic evidence suggest that this species is most closely related to the common tern (Sterna hirundo) and Antarctic tern (S. vittata); a few reports of hybridization with common, roseate (S. dougallii) and Forster's terns (S. forsteri) exist (Hatch 2002).